One proposed type of hybrid vehicle has a clutch motor, which includes an inner rotor connecting with a crankshaft of an engine and an outer rotor connecting with a drive shaft linked with an axle, and an assist motor that outputs power to the drive shaft (see, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 9-207622). When a target vehicle speed is set for a constant speed drive (auto cruise), the hybrid vehicle calculates energy to cancel a difference between the target vehicle speed and an observed vehicle speed, increases energy output from the engine by the calculated energy, and increases the torque of the assist motor by a torque corresponding to the increased output energy. The hybrid vehicle is thus driven stably at the target vehicle speed. An insufficiency of energy due to a delay of a response to the increase in energy output from the engine, that is, energy required to increase the torque of the assist motor, is supplied from a battery.
The automobile, as well as the hybrid vehicle, often uses a map to set a vehicle driving force corresponding to an accelerator opening, which reflects a driver's accelerator step-on action. Even in the constant speed drive of the automobile at the target vehicle speed set by the driver, the accelerator opening may be set according to the observed vehicle speed and the target vehicle speed. The control in the standard drive is then applicable to the control after setting the accelerator opening in the constant speed drive. The hybrid vehicle controls the internal combustion engine and the motor based on the accelerator opening. It is accordingly desirable that the control in the constant speed drive shares a greater portion of the control in the standard drive. The map used to set the vehicle driving force corresponding to the accelerator opening typically includes a dead zone where the vehicle driving force is kept unchanged to 0 with a variation in accelerator opening. This prevents the vehicle driving force from frequently fluctuating between the positive values and the negative values. In the constant speed drive on a downslope with the map including the dead zone, the accelerator opening varied according to the observed vehicle speed and the target vehicle speed may enter the dead zone of the map. In this case, the vehicle speed is not stabilized to the target vehicle speed.